Field of the Invention
Embodiments of the present application relate to the art of a drive motor used as a prime mover of automobiles, and especially to a motor drive unit having a power transmission device between the drive motor and drive wheels.
Discussion of the Related Art
PCT international publication WO 2015/008661 describes one example of a motor drive unit of this kind. The drive gear unit taught by WO 2015/008661 as a torque vectoring device comprises a differential unit for distributing torque delivered from a drive motor to right and left drive wheels, and a differential motor for controlling a torque split ratio to the drive wheels. The differential unit comprises a pair of single-pinion planetary gear units, and sun gears of the planetary gear units are connected to both ends of a rotary shaft. An input gear is fitted onto an intermediate portion of the rotary shaft, and torque of the drive motor is applied to the input gear. Ring gears of the planetary gear units are connected to each other through a torque reversing device so that the ring gears are rotated in opposite directions. The differential motor is connected to one of the ring gears, and drive wheels are connected the carriers of the planetary gear units.
A driving performance of an electric vehicle may be improved by using the torque vectoring device taught by WO 2015/008661 as a motor drive unit. The motor drive unit of this kind may serve as an inboard brake by incorporating a brake device into the motor drive unit to apply brake torque to the drive wheels. To this end, for example, an electromagnetic brake may be used in the motor drive unit. In this case, a hydraulic system and reinforcements such as a brake caliper and so on may be omitted, and hence the motor drive unit may be downsized and lightened. In addition, an unsprung load of the vehicle may be reduced.
In order to selectively apply a braking torque to the vehicle, an electromagnetic brake adapted to generate a braking torque by energizing an electromagnetic coil is suitable for the use as the inboard brake. However, the electromagnetic brake cannot maintain a braking torque to stop a motion of a vehicle during parking while turning the power off.
Alternatively, an electromagnetic brake adapted to generate a braking torque utilizing elastic force of a spring may also be used as the inboard brake. In this case, a braking torque may be applied to the vehicle even when a coil of the electromagnetic brake is unenergized. In this case, however, the braking torque is applied to the vehicle immediately even if a current supply to the coil is stopped by some sort of failure during propulsion and consequently the vehicle is decelerated abruptly. Thus, the electromagnetic brake of this kind is unsuitable to be used as the inboard brake in the motor drive unit of automobiles.